1. Field of the Invention
The present invention relates generally to drilling subterranean formations with rotary bits and, more specifically, to superabrasive cutting elements particularly suitable for drilling plastic formations, rotary bits so equipped, a drilling system employing such bits, and a method of drilling employing such bits.
2. State of the Art
Superabrasive cutting elements have been employed for many decades in the drilling of subterranean formations, especially for the production of hydrocarbons. Natural diamonds were first employed, but during the last twenty years, synthetic, polycrystalline diamonds, commonly referred to as polycrystalline diamond compacts, or PDCs, have become the superabrasive of choice for drilling most formations. A typical, state-of-the-art PDC cutting element exhibits a disk-like polycrystalline diamond "table" having a substantially flat, circular cutting face and formed in an ultra-high temperature, ultra-high pressure process onto a preformed, supporting substrate of cemented or sintered tungsten carbide (WC). Traditionally, a PDC cutting face has been lapped to a smooth finish. The PDC cutting elements as described are fixed to so-called rotary "drag" bits used to shear material from a rock formation being drilled by contact of the cutting elements with the formation under rotation and applied weight on bit (WOB).
While PDC cutting element-equipped bits have proven very effective in cutting certain formations, other formations, particularly some of those which fail plastically, have presented a substantial obstacle to effective and efficient PDC drag bit drilling due to the tendency of cuttings from those formations to adhere to the cutting faces of the cutting elements. For example, PDC cutting elements shear some shales with little problem, generating formation cuttings or "chips", which can be removed from the bit face using conventional bit hydraulics. As pressure stresses increase with well bore depth, however, a formation becomes more plastic and requires different bit cutting mechanics to cut efficiently. Such difficult formations include, by way of example, highly pressured or deep shales, mudstones, siltstones, and some limestones. The problem is exacerbated as the density of the well bore fluid increases.
Shale and other ductile formations tend to flow more easily at stress and thus conform, and adhere more strongly, to surfaces they contact. As a result, shear stress necessary to displace a cutting from the cutting face of a cutting element increases significantly. In fact, it is believed that the shear stress required to displace a formation cutting from a cutting face may be higher than the stresses initially required to shear the cutting from the formation. Formation cuttings adherence to the cutting face thus may result in a relatively stationary mass of formation material built up immediately ahead of the cutting edge at a periphery of the cutting face. This mass comprises a tough, solidified agglomeration of formation cuttings, initiated through shear enhanced compaction and hydration of the formation material. As a result, instead of contact between the cutting face and the uncut formation comprising a point or line (depending on the degree of wear of the cutting element) at the peripheral cutting edge of the cutting face, the cuttings mass, sometimes referred to as a built up edge (BUE), presents a very dull or blunt geometry to the formation, resulting in a much larger area of contact with the uncut formation material, compressing the formation material and increasing the effective stress of the formation being cut. Further, the presence of this mass moves the cutting action away from and ahead of the cutting edge, altering the failure mechanism and location of the cutting phenomenon so that cutting of the formation is actually effected by the mass itself, which obviously is quite dull, rather than by the cutting edge as intended. Thus, the presence of a BUE hinders the performance of the cutting element and lowers the rate of penetration (ROP) of the bit on which it is employed.
In recent years, a substantial and commercially successful solution to the chip-to-cutting face adherence problem has been developed. U.S. Pat. Nos. 5,447,208 and 5,653,300, assigned to the assignee of the present invention and incorporated herein for all purposes by this reference, disclose and claim the use of superabrasive (also sometimes termed "superhard") cutting elements exhibiting cutting faces or cutting face portions which are polished or otherwise worked or formed to an extremely high degree of smoothness, including to a mirror-like finish. Such cutting elements have demonstrated a superlative ability to resist adherence of the aforementioned plastic formation cuttings to the cutting face, thus avoiding the BUE comprising a mass of formation material located ahead of the cutting edge, and promoting cutting adjacent the cutting edge itself.
While the mechanism by which cuttings adherence is not fully understood, it is believed to be largely attributable to a substantial (on the order of 50% or more in comparison to conventional, lapped cutting elements) reduction in the coefficient of friction of the cutting face portion which exhibits the aforementioned extremely smooth finish. This significant reduction in friction between the cutting face and formation, and consequent reduction in cuttings adhesion, reduces the shear stress of or resistance to movement of formation cuttings across the cutting face, and thus the normal as well as tangential forces required for a specified depth of cut in a given formation. In addition, the compressive rock strengthening effect that often occurs in front of a cutting element due to the presence of the BUE is avoided. The reduction in friction has even, surprisingly, been demonstrated to overcome the phenomenon of cuttings adherence to the cutting face of a cutting element due to the presence of a positive pressure differential on a formation cutting arising out of the presence of greater well bore pressure on the outside, or exposed face, of the cutting, than ambient formation pressure present on the side of the formation cutting lying adjacent the cutting face across which it is traveling. In extensive field use, the polished cutting face PDC cutting elements have also demonstrated a marked superiority in rate of penetration (ROP) even in non-plastic formations, as well as in durability and resistance to wear during the drilling process.
However, field experience with polished cutting elements has also demonstrated a new difficulty in the drilling of some plastic formations, even with the above-described cutting action occurring proximate the actual cutting edge of the cutting element, rather than ahead of the cutting edge. This edge-cutting action results in long, ribbon-like cuttings akin to a cutting taken by running a knife across a cake of soap. In certain formations, particularly those such as shales including a significant volume of reactive clays, cuttings from the various cutting elements on the cutting face of a typical, multi-cutting element PDC bit may quickly agglomerate into a semi-solid mass which must literally be extruded through the junk slots on the gage of the bit, thus defeating the bit hydraulics and preventing their effective removal up the well bore annulus to the surface. This junk slot clogging with an agglomeration of cuttings in turn foments a build-up of subsequent cuttings above (as the bit is oriented during drilling) the agglomeration on the bit face, until the bit generates a mass of agglomerated cuttings covering the bit face. At this point the bit "balls up" and ceases drilling when the cutting elements are no longer cutting the formation, but riding on the agglomerated cuttings mass.
Chip breakers have been used to fragment the long, ribbon-like cuttings into shorter segments. Additionally, hydraulic design and drilling fluid flow volume of state-of-the-art bits have been enhanced in order to move the cuttings more efficiently to and through the junk slots. However, in many instances polished PDC cutting element drag bits can still literally out-drill their ability to dispose of formation cuttings. As a result, rotary speed and weight on bit may be undesirably limited in order to reduce the volume of formation cuttings to a level commensurate with the bit's ability to move the cuttings away from the bit face and up the annulus. Consequently, ROP is lessened, and rig time increased, to drill an interval through formations through which polished PDC cutting element drag bits are otherwise ideally suited. Stated another way, in such situations, ROP becomes a function of the rate of extrusion of the agglomerated cuttings mass through the junk slots of the bit.
The required use of water-based, rather than oil-based, or water-in-oil invert emulsion drilling fluids in environmentally-sensitive or otherwise highly regulated drilling locations may also severely limit the ROP of PDC-equipped drag bits, particularly in deeper shales. Many, if not most, water-based drilling fluids fail to prevent or even substantially retard the above-referenced cuttings agglomeration problem, which is attributable to the presence of reactive clays in such formations. Reactive clays may generally be categorized as those which change atomic structure or physical properties in the presence of a water-based drilling fluid system, leading to the above-referenced cuttings agglomeration problem.
Further, conventional wisdom regarding PDC cutting element design has dictated that the cutting edge of such a cutting element (including so-called polished cutting elements) be beveled or chamfered to a noticeable degree, typically to at least 0.010 inch looking face-on and perpendicular to the cutting face and most commonly at a 45.degree. angle to the longitudinal cutter axis. This chamfering or beveling has been shown to be effective in tougher or harder formations, or lenses, in order to reduce chipping and potential fracture of the superabrasive table until the cutting element begins to form a wear flat along the line of contact with the formation, extending the line to a surface of contact transverse to the direction of travel of the cutting element as it moves with rotation and downward movement of the drag bit to which it is secured. Unfortunately, chamfers or bevels of a magnitude sufficient to reduce damage to the superabrasive tables also result in a relatively blunt cutting element presentation to the formation. This type of cutting edge geometry actually increases the stress required to fail the formation rock opposite the chamfer, particularly in rocks which fail plastically. Therefore, PDC cutting element cutting efficiency is not optimized, even with an extremely smooth, polished cutting face according to the '208 patent.
Thus, the state of the art has failed to date to provide a means and method for taking full advantage of polished cutting face cutting elements in formations for which they are particularly suited.